1. Field of the invention
The present invention relates to an apparatus for air conditioning the interior of an electrically-powered motor vehicle.
2. Prior art
An air conditioning system for a conventional internal combustion engine-driven motor vehicle is constructed as shown in FIG. 1 (a). A compressor 1 for compressing the coolant is driven by the engine; heat is discharged by the outside air heat exchanger 2 and the fan 3 for the outside air heat exchanger, and the coolant is condensed to a liquid. The condensed coolant is then transferred through an expansion valve 4 to the interior air heat exchanger 5 by which the air inside the passenger compartment is cooled, dehumidified, and the coolant vaporized by the interior air heat exchanger fan to cool the inside of the vehicle.
The passenger compartment is heated using the heat of the engine passing through a hot water heating coil.
An air conditioning apparatus for an electrically-powered motor vehicle cooling can be provided by an essentially similar system, but the engine heat is not present and therefore cannot be used for heating. It is therefore necessary to modify the section indicated by the dotted line in FIG. 1 (a) as shown in FIG. 1 (b) to form a heat pump heating system. A four-way switching valve 7 downstream from the compressor 1 reverses the coolant flow so that the heat from the interior air heat exchanger 5 is discharged into the passenger compartment and the coolant is condensed. The condensed coolant is then passed through the expansion valve 4 to the outside air heat exchanger 2, which then cools and dehumidifies the air outside the vehicle so that the coolant absorbs heat and is again vaporized.
It should be noted that a heat pump heat system using engine waste heat in an air conditioning system for an internal combustion engine-driven vehicle has also been described in Japanese patent Application Publication (unexamined) H3-90430.
Because the coolant in the outside air heat exchanger reaches a low temperature when this system is operating in the heat pump heating mode, the moisture absorbed from the air tends to condense as frost or ice on the outside air heat exchanger, thus preventing the outside air heat exchanger from exchanging heat with the air. It is therefore necessary to provide a defrost cycle for the outside air heat exchanger.
In a residential room air conditioner this defrost cycle is accomplished by switching the four-way switching valve to the cooling mode position, effectively raising the temperature of and defrosting the outside air heat exchanger. The outside air heat exchanger fan is also stopped during the defrost cycle because defrosting is not possible if the outside air heat exchanger is exchanging heat with the air. This same cycle is applied for outside air heat exchanger defrosting in a motor vehicle with a heat pump heating mode.
In an electrically-powered motor vehicle, however, a supplemental heat source must be provided because waste heat from the engine is not available. The natural air flow of vehicle movement causes air to pass through the outside air heat exchanger even when the outside air heat exchanger fan is stopped. The outside air heat exchanger therefore continues to exchange heat with the air, and defrosting in the heat pump heating mode becomes impossible.